Glacial Sediment Transport
Great strides have been made in recent years in collaborative interpretation of seismic data from the Antarctic margin (through the ANTOSTRAT initiative: see Cooper et al., 1994; 1995). Together with the simplicity of the modern Antarctic glacial regime (compared with that of the Arctic), these data have led to the rapid emergence and application of a unifying model of glacial sediment transport and deposition (Alley et al., 1989; Larter and Barker, 1989; Bartek et al., 1991; Cooper et al., 1991; Kuvaas and Kristoffersen, 1991). Briefly, almost all ice transport to the ice-sheet margins takes place within broad, rapidly moving ice streams. Rapid flow is enabled by low friction basal conditions, the main source of which is the existence of an overpressured and undercompacted, unsorted, shearing basal till. The necessary shear ensures that ice transport is accompanied by till transport, and virtually all of the transported till is melted out/dropped/deposited very close to the grounding line, where the ice sheet becomes ice shelf before calving into icebergs and drifting north. The ice stream, therefore, essentially erodes and transports inshore of the grounding line and deposits directly offshore in a high-latitude analogue of the low-latitude subaerial erosion/shoreline/marine sedimentation system. Further, the grounding line advances and retreats under the influence of upstream ice provision and basal sediment supply‹and sea-level change‹that are all related to climate. The very large prograded sediment wedges beneath the Antarctic margin were developed during a series of glacial maxima, when the ice sheet was grounded all the way to the continental shelf edge (Fig. 5).

The glacial sedimentation regime has other characteristics. Progradation is usually focussed into broad "trough-mouth fans" opposite the main ice streams, and the shelf is overdeepened (generally to 300-600 m depth, but in places much deeper) and inward-sloping. Continental slopes are often steep, and in places turbidity-current transport of the unstable component of slope deposition (with down-current deposition of suspended fines) has produced large hemipelagic sediment drifts on the continental rise (Kuvaas and Leitchenkov, 1992; Rebesco et al., 1996; Fig. 6). Sediment supply to the slope and rise is highly cyclic, with large quantities of unsorted diamicton deposited during glacial maxima and very little deposited during interglacial periods.

Three depositional environments are recognized: shelf topsets and slope foresets of the prograded wedge, and proximal hemipelagic drifts on the continental rise. Of these, the shelf record is potentially the least continuous. There, sediment is preserved mainly as a result of slow subsidence from cooling and from flexural response to the topset and foreset load, and the sediment is prone to re-erosion during the next glacial advance. The topsets tend to mark only the major changes in glacial history, so that the more continuous foreset record is an essential complement. The proximal rise drifts may not always be present and are as yet sparsely sampled, but potentially contain an excellent record, closely related to that of the upper slope foresets from which they are derived. Existing seismic data and drill sites from around Antarctica have demonstrated the coarse (but not as yet the fine) scale climate record in continental rise sediments and the likely climatic sensitivity of margin wedge geometry (Barker, 1995), and have revealed the partial nature of the shelf topset record (Hayes, Frakes, et al., 1975; Barron, Larsen, et al., 1989).

The continental shelf is an area of high biogenic productivity during interglacial periods. Although long-term sediment preservation on the shelf is limited because of the erosional effects of grounded ice sheets during subsequent glacials, biogenic interbeds will be preserved within sequence groups composed mainly of thick glacial diamicton topsets and foresets. In addition, glacially eroded deeps can preserve expanded Holocene sections that may be continuous and essentially biogenic, provided the ice-sheet grounding line is sufficiently remote that ice-rafted debris is minor or absent and the section is sufficiently protected from bottom current action. Such sections can provide a record of decadal and millennial variability that can be compared with records from low latitudes and the ice sheet itself. This environment is available on the inner shelf of the Antarctic Peninsula (Domack and McClennen, 1996) and will be sampled during Leg 178.

Regional Features of Antarctic Glaciation
Different parts of Antarctica have had different glacial histories. The present Antarctic ice sheet comprises an East Antarctic component grounded largely above present sea level and a West Antarctic component grounded largely below sea level. Marine-based (West Antarctic) ice sheets are considered less stable. There is evidence from around Antarctica that, although East and West Antarctic climates were coupled in the past, changing approximately in phase, the climate of West Antarctica (including the Antarctic Peninsula) has varied around a consistently warmer baseline. Although East Antarctic glaciation extends to 35 Ma or earlier, West Antarctic glaciation probably began more recently, during generally colder times. Further, there is strong evidence that Northern Hemisphere glaciation has been the main contributor to global sea-level change over the past 0.8 m.y. and probably 2.5 m.y., and has therefore partially driven the more subdued changes in Antarctic glaciation. Another significant local control may have been the Transantarctic Mountains, which probably attained much of their present elevation and influence on the East Antarctic ice sheet during late Cenozoic time.

Antarctic Peninsula Region
Tectonic Influences On Sedimentation
The tectonic setting of the Antarctic Peninsula is unusual, but straightforward. Subduction of the Pacific ocean floor that had occurred for 150 m.y. or more ended with collision of a (Phoenix Antarctic) ridge crest at the trench, earliest (~50 Ma) in the southwest and latest (6-3 Ma) in the northeast. In the far northeast, the surviving South Shetland Trench and extensional Bransfield Strait form a modern complexity that does not concern us here. Generally, the effects of collision have included (1) some terrigenous sedimentation in and beyond the ridge crest in the last 2-3 m.y. before collision and (2) uplift of the margin soon after collision followed by slow subsidence, leading to a hiatus in terrigenous sediment supply to the rise in that particular collision segment for a few million years after collision. Collisions occurred well before the onset of glaciation in the southwest, but not in the northeast. In the northeast, this provides a useful constraint on the maximum age of glacial sediments (they overlie ocean floor of known age), but also threatens interference between tectonic and glacial events. For the older glacial history it is prudent to avoid the northeast area of the margin.

Antarctic Peninsula Glacial Sedimentation

The ultimate aim of the four or five linked ANTOSTRAT drilling proposals is to provide an estimate of the variation in size of the Antarctic Ice Sheet through the Cenozoic. Each ANTOSTRAT proposal is focussed on the particular contributions its region might make toward understanding Antarctic glacial history. A single region does not offer the best opportunities for drilling in all respects. The particular value of drilling on the Antarctic Peninsula is made clear below, in terms of the main influences on glacial sedimentation.

Onshore evidence of Eocene glaciation on the South Shetland Islands (northern Antarctic Peninsula) has been published (see Birkenmajer, 1992), but this conflicts with other evidence of regional climate. Generally, it is considered that the Antarctic Peninsula can provide a high resolution record of glaciation back to perhaps 10 Ma. To go back farther could involve entanglement with the tectonics of ridge-crest collision, making this a problem rather than an asset. However, because of the Antarctic Peninsula's more northerly position, its glacial history is shorter than East Antarctica's. The record before 10 Ma may be largely nonglacial, or may reveal a stage of valley glaciation lacking regular ice-sheet extension to the continental shelf edge.

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